A known after-treatment system for exhaust gas passing through an exhaust system of a diesel engine comprises a diesel oxidation catalyst (DOC) associated with a diesel particulate filter (DPF). The combination of these two exhaust gas treatment devices promotes chemical reactions in exhaust gas and traps diesel particulate matter (DPM) as exhaust flows through the exhaust system, thereby preventing significant amounts of pollutants such as hydrocarbons, carbon monoxide, soot, SOF (soluble organic fraction), and ash, from entering the atmosphere.
A DPF is regenerated from time to time in order to maintain particulate trapping efficiency. Regeneration involves creating conditions that will burn off trapped particulates whose unchecked accumulation would otherwise impair DPF effectiveness and/or engine performance.
At times a vehicle may be operating in a way that inherently burns off trapped materials in a DPF. This naturally occurring regeneration is sometimes called “passive” regeneration. Passive regeneration generally has little or no significant effect on either fuel economy or vehicle driveability.
When trapped materials accumulate in a DPF faster than they are passively removed, they eventually reach a level, often expressed as a percentage, that calls for the DPF to be regenerated. A regeneration initiation and control strategy in the engine controller forces regeneration. Such a forced regeneration is sometimes referred to as “active” regeneration.
A typical regeneration initiation and control strategy controls air and fuel management systems in a manner that elevates engine exhaust gas temperature to one that is high enough to burn off trapped DPM. One way to elevate exhaust gas temperature is by post-injection of fuel.
Active regeneration is less efficient than passive regeneration because extra fuel is used to elevate the exhaust gas temperature. Apart from causing some reduction in vehicle fuel economy, active regeneration may also have an effect on vehicle drivability that if significant enough, should be addressed to assure customer satisfaction.